Mr. Yang arranged meetings for Mr. Adelson with senior Chinese officials, acted as a frontman on several ambitious projects for Mr. Adelson’s company, the Las Vegas Sands Corporation, and intervened on the Sands’s behalf with Chinese regulators. Mr. Yang even had his daughter take Mr. Adelson’s wife, Miriam, shopping when she was in Beijing.

“Adelson and I had a good relationship,” Mr. Yang said in a recent interview in Hong Kong. “He should thank me.”

Mr. Yang joined the Sands in 2007 as the company worked to protect its interests in Macau, where its gambling revenues were mushrooming, and pressed ahead with plans for a resort in mainland China. Boasting of ties to the People’s Liberation Army and China’s state security apparatus, Mr. Yang was hired for his guanxi, that mixture of relationships and favors that is critical to opening doors in China, according to former executives.

But today, Mr. Yang, along with tens of millions of dollars in payments the Sands made through him in China, is a focus of a wide-ranging federal investigation into potential bribery of foreign officials and other matters in China and Macau, according to people with direct knowledge of the inquiries.